1. Field of the Invention
The present invention relates to a receiving apparatus, an imaging apparatus, and a receiving method, and, more particularly to a technique for controlling a gain such that a signal level of an input signal transmitted through a communication cable is converted into a fixed level range.
2. Description of the Related Art
In the past, in a broadcasting station, plural cameras are connected to a camera control unit (hereinafter simply referred to as a control unit) via a cable for transmitting an analog signal. Video signals and sound signals captured by the cameras are transmitted to the control unit through the cable. Return signals for instruction and confirmation are transmitted from the control unit through the cable.
As the cable for connecting the cameras and the control unit, in general, a coaxial cable is used because the coaxial cable is relatively low in cost. For example, in a TRIAX system adopted in many broadcasting stations, power supply is performed and video signals, sound signals, command signals, and camera return signals are transmitted with a frequency division multiplexing wave by using one coaxial cable.
In recent years, signals of an HDTV (High Definition Television) system having a large image information amount compared with the system in the past have been spread. The coaxial cable is also used for transmission of such signals having a large image information amount. Originally, it is desirable to use an optical fiber for a signal transmission channel for transmitting an HD signal in a wide band to the control unit. However, large cost and time are necessary for installing the optical fiber. Therefore, under the present situation, replacement with the optical fiber is not advanced.
Further, in recent years, in order to improve mobility and usability, longer cable length is requested for the cable for connecting the control unit and the cameras. In this way, it is demanded to extend the distance of the existing coaxial cable and use the coaxial cable for transmission of signals having a large image information amount such as the HD signal. However, since the analog transmission system has the problems explained below, it is not easy to realize the demands.
As a first problem, as the distance of the cable is extended, a signal level falls because of an increase in a loss of the cable and an S/N value at a reception end may be deteriorated in proportion to the cable distance. As a second problem, there is a transmission loss characteristic that the signal level is attenuated as a frequency of an analog signal increases. FIG. 14 is a graph of a relation between a frequency and an attenuation amount of a signal in the coaxial cable. The abscissa indicates the frequency (MHz) and the ordinate indicates the attenuation amount (dB). In FIG. 14, an example of an attenuation characteristic obtained when the cable distance is 1 km is shown.
As it is evident from FIG. 14, the attenuation amount increases as the frequency increases. For example, when a signal of 10 MHz and a signal exceeding 100 MHz are compared, it is seen that a difference between signal levels of the signals is equal to or more than 90 dB. Therefore, a √f (f is a frequency) cable equalizing circuit and a gain control circuit (automatic gain control (AGC)) circuit for outputting, at a fixed level, reception signals input at various levels are necessary according to the cable distance.
For example, JP-A-2008-029000 discloses a gain control circuit that switches to use plural gain amplifiers according to the level of an input signal to suppress fluctuation in a signal loss caused in a process of transmission through a communication cable and enable extension of communication cable length.